Desorption rate limitation in the extraction of organic molecules from unsaturated soils during soil venting operations

A microscale mathematical model is developed to analyze the desorption rate limited extraction of volatile organic compounds in a soil column. By solving the diffusion equation in the liquid layer around the soil particles and incorporating the effects of slow desorption from the soil surfaces, it is found that the concentration of organics in the effluent is eventually given by the sum of two separate contributions. The first represents the removal of the compounds initially dissolved in the liquid layer by diffusion, providing an exponentially decaying concentration in the effluent. The second represents the removal of the organics, initially adsorbed onto the solid surfaces, by slow desorption producing the long-time tail which has been observed in the effluent concentration during soil vapor extraction. This model was applied to two bench-scale columns packed with soil contaminated by volatile organic compound (VOC) contaminated soil. The simplified results of the mathematical model allow such parameters as the effective diffusion length, area of gas-liquid contact, the desorption rate constant and the equilibrium partition coefficient, for trichloroethylene in the two soil columns to be determined.